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<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>

    function flatten(arr) {
      let res = arr.toString().split(',')
      return res
      // console.log(res);
    }

    const res = flatten([1, 2, [3, 4, [10, 20, [100, 200]]], 5])


    console.log(res) // [1, 2, 3, 4, 10, 20, 100, 200, 5]
  </script>
</body>

</html>